In power transmission and distribution systems, rapid current rises can occur, such as those occurring during those short-circuit conditions. In order to protect the high-voltage power lines against the dynamic and thermal stresses which accompany the rapid current rises, the line must be electrically opened or cut-off before the short-circuit current has reached its peak value if the line is carrying alternating current, or before the line has reached its final value if the line is carrying direct current. The cut-off time required, which depends upon the frequency of the alternating current and on the inductance, capacitance and resistance of the power line, should not exceed a few milliseconds. Such rapid cut-off times, however, cannot be obtained with mechanically or magnetically actuated switches in medium-voltage and high-voltage power networks. Therefore, switches have been developed which are actuated by blasting.
One conventional type of blast-actuated switch contains a hollow bridge conductor which electrically connects two external connections. A blasting cap is included in the hollow region of the conductor, approximately half way between each of the external connections. The blasting cap includes two wires which may be connected to an electric ignition device. The blasting cap is ignited, thereby blasting the bridge conductor with an explosive force. To prevent the scattering of fragements of the material which make up the bridge conductor at the time of blasting, the bridge is slotted in the longitudinal direction. Conductor webs, which are defined by the slots, each include a notch or a soldered joint at their longitudinal centers. Blasting the notches or soldered joints results in the webs being bent back to form rosettes around the associated external connection.
The energy stored within the inductance of the high-voltage power line will cause a steep rise in voltage across the external connections when the bridge conductor is blasted. The voltage may rise to multiples of the operating voltage of the line. In order to prevent the rise in voltage across the separated ends of the bridge conductor from arcing and thereby delaying the cut-off process, a fusible wire is connected in parallel with the bridge conductor. The fusible wire is embedded in quenching sand. Current will flow through this wire in parallel with the separated ends of the bridge conductor and therefore arcing across the separated ends will be surpressed. As soon as the rising voltage exceeds a predetermined value, the current flow through the fusible wire causes the wire to immediately melt, and the quenching sand prevents any further arcing.
The gap between the separated ends of the bridge conductor possesses only a limited dielectric strength because the webs, bent back to form rosettes around the associated external connections, often have sharp edges. The density of the lines of flux and the gradient of the electric field potential in the vicinity of the sharp edges will be high. As a result, arcing may occur between the separated ends of the bridge conductor.
It is therefore an object of the present invention to provide a high-voltage power switch in which the electric field between the two external connections is shielded from the effects of sharp edges formed on the separated ends of the bridge conductor when the bridge conductor is blasted.